user_stats call errors if one ingester is down

[bboreham]

It's trying to aggregate rates over all ingesters, so when one is down the numbers will be lower, but perhaps there is another way to indicate this other than a 500 error.

[bboreham]

Distributor.UserStats() calls forAllIngesters() with reallyAll flag, which is what causes it to error if one is down.

[bboreham]

The endpoint is registered on Querier, so we may want to refactor the code so it isn't implemented inside Distributor.